TIG on closed object

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I was finishing a project this morning and ran into a problem.I was TIGing on a closed aluminum object and was almost finished when everything just went crazy. I'm pretty sure it had to do with the heated air inside trying to force it's way out through the weld which contaminated it and the aluminum was melting instead of welding properly. It looked like I had dipped the tungsten with black soot, etc., and the base metal just sort of opened up really ugly.I changed from a 6 cup to an 8, turned up the argon, and finally got it closed by starting away from the area and feeding 3/32 filler into the puddle while quickly moving into the open spot. Then I had to grind it back down to the surface level. I drilled 2 small vent holes in the bottom so I could get the main surface closed, so I still need to close them back.My question is, am I right about the heated air from inside causing the problem or is there something else going on? What could I do to avoid that from happening, and how should you get it closed? I thought about putting it in the freezer and trying to weld it closed while it was cold, but didn't try that. Other suggestions or ideas?Burt _____________________Miller Syncrowave 250Millermatic 211Miller 375 Plasma Cutter Hobart Handler 12010FtDrillBit.com
Reply:wb4rt, depending on size of the enclosed volume to be sealed, just take a rag wet with acetone and slosh the sides of the object to seal, give it a few seconds to evaporate and feel it with your hand, when its cool to the touch, the holes will be pulling in (vacuum) and will seal.If the volume is large enough wetting with water will do this fine, smaller objects- acetone works great.cheers,Kevin Morin
Reply:Yes hot gas on the interior escaping. Solutions:weld all but the last tiny bit (experience defined bit) after it cools go back it finish. put an escape hole somewhere else that won't be getting hot during the welding process weld it last.